Some things to try:
Manually search for another network
Reboot the phone
Turn off 5g in network settings
If you have an iPhone change your data APN to "mobile.o2.co.uk"
If still not working then contact O2 to remove the Roaming bar on your account.
If you message O2 on Facebook ( https://o2uk.co/O2CFB) , Twitter ( https://o2uk.co/O2CTW) or Instagram ( https://o2uk.co/O2CIG) , they should be able to help you with this.